The present invention relates to an apparatus for the condition monitoring the integrity of fasteners and fastened joints.
One of the problems faced by aircraft designers relates to the loosening of fasteners such as rivets and bolts used on aircraft structures leading to fretting as a result of movement of the fastener or the initiation of cracking and over stressing of adjacent fasteners. Cracks or corrosion formed under fastener heads or in layers of sheet material below a surface layer can also cause fasteners to become loose.
Cracks in fasteners of pressure hulls or fuel containment areas are of great concern due to the possibility of rapid growth and catastrophic structural failure.
The present practice is to merely detect the loosening or misalignment of a fastener by detecting leaks around the fastener utilizing vacuum techniques. This method can be applied to every fastener or to selected fasteners. However, the above method does not allow quick comparative measurement.
Schupack et al U.S. Pat. No. 4,979,390 discloses a method and apparatus for testing the relative permeability of materials, and thus could be used for condition monitoring of the integrity of fasteners and fastened joints. However the method does not allow quick comparative measurement as it is not a rate measurement (ie, it is dependent on elapsed time) and further, would be structurally damaging to sheet metal structures due to localised high differential pressure forces across the test area.
It is an object of the present invention to provide an apparatus for the condition monitoring of the integrity of fasteners and fastened joints which attempts to alleviate the disadvantages of the above described prior art techniques.
According to the present invention there is provided an apparatus for the condition monitoring of the integrity of fasteners and fastened joints in a structure including at least:
a substantially fluid impervious pad for placement over a head of one or more fasteners adjacent a surface of the structure, said pad having a first surface for contacting said structure, a portion of said first surface configured to define a region facilitating a free flow of fluid across said first surface between said pad and said structure when said first surface is in contact with said structure covering said one or more fasteners;
a substantially constant vacuum source;
a high fluid flow impedance coupled between said constant vacuum source and said region; and,
means for measuring differential pressure across said high fluid flow impedance.
Preferably, said pad is collapsible to the extent that said first surface can substantially conform to the contour of the head of said fasteners when said pad is in contact with said structure and the region is in fluid communication with said constant vacuum source.
Preferably, said apparatus further includes a bypass conduit connected in parallel across said high impedance between said region and said constant vacuum source, and first valve for selectively opening and closing fluid communication through said bypass conduit.
Preferably, said apparatus further includes a second valve for selectively opening and closing fluid communication through said high fluid flow impedance.
Preferably, said first surface is formed with a plurality of protrusions for contacting said fasteners and/or structure, said protrusions shaped so that gaps exist between mutually adjacent protrusions, whereby said region is in the form of a cavity defined by said gaps.
Preferably, said apparatus includes sealing means for providing a fluid tight seal about a periphery of said portion of said first surface.
Preferably, said sealing means includes an elastomeric strip provided in said pad about the periphery of said portion of said first surface.
Preferably, said sealing means further includes a channel formed in said elastomeric strip; and, said apparatus further includes means for selectively placing said channel in fluid communication with an independent vacuum source.
In an alternative embodiment, said sealing means includes means for releasably mechanically holding said elastomeric strip in sealing engagement with the surface of said structure.